1. Field of the Invention
The present invention relates to a thin ferroelectric film element and to a method for manufacturing the same. The thin ferroelectric film element of the present invention can be used for a memory element (e.g., a capacitor), a pyroelectric sensor (e.g. an infrared linear array sensor, a supersonic sensor), and a piezoelectric element (e.g., an optical modulator).
2. Description of the Related Arts
Thin ferroelectric films have numerous functions such as spontaneous polarization, high dielectric constant, electrooptical effect, piezoelectric effect and pyroelectric effect, and hence are applied to a wide range of device developments. For example, thin ferroelectric films are applied to infrared linear array sensors by utilizing their pyroelectricity, to supersonic sensors by utilizing their piezoelectricity, to optical modulators of a waveguide type by utilizing their electrooptical effect, and to capacitors for DRAMs and MMICs by utilizing their high dielectric constant.
Especially among these various application device, there has been a development of ferroelectric non-volatile memories (FRAMs) that are highly dense and can operate at a high speed by combination with a semiconductor memory technique in accordance with the recent progress of thin film formation technique. Non-volatile memories incorporating a thin ferroelectric film are under extensive research and development for practical use not only as a replacement for conventional non-volatile memories but also as a memory that can be substituted for SRAMs and DRAMs owing to their properties such as high speed writing/reading, low-voltage operation, and high endurability in writing/reading.
In conducting these device developments, a material is required that has a large residual polarization (Pr), low coercive field (Ec), low leakage current, and endurability in repetition of polarization inversion. Further, it is preferable that the above properties are achieved with a thin film of 2000 .ANG. or less so as to decrease the operation voltage and to conform to the fine processing of semiconductors.
Here, oxide materials having a perovskite structure such as PZT (lead titanate zirconate, Pb(Ti.sub.x Zr.sub.1-x)O.sub.3) have been mainly used as ferroelectric materials for these purposes. However, in the case of a material such as PZT containing lead as its constituent element, lead tends to evaporate at the time of forming a film due to high vapor pressure of lead or its oxide, whereby defects or, in extreme cases, pinholes are generated in the formed film. This may increase the leakage current and, when the polarization inversion is repeated, this may cause a fatigue phenomenon in which the spontaneous polarization decreases. Particularly, in view of substituting ferroelectric nonvolatile memories for FRAMs, it must be ensured, with respect to the fatigue phenomenon, that the characteristics remain unchanged even after 10.sup.15 times repetition of polarization inversion. Accordingly, the development of a thin ferroelectric film without a fatigue has been desired.
Meanwhile, a research and development of bismuth layered-structure compound materials has been recently taking place as a ferroelectric material for FRAMs. The bismuth layered-structure compound materials were found by Smolenskii and others in 1959 (G. A. Smolenskii, V. A. Isupov and A. I. Agranovskaya, Soviet Phys. Solid State, 1, 149(1959)), and were subsequently examined in detail by Subbarao (E. C. Subbarao, J. Phys. Chem. Solids, 23,665(1962)). Recently, Carlos A. Paz de Araujo and others have found that thin films of bismuth layered-structure compound are suitable for application to integrated circuits of ferroelectrics and high dielectrics, and have reported an excellent fatigue property that the characteristics remain unchanged even after 1012 times repetition or more of polarization inversion (International Application No. PCT/US92/10542).
The bismuth layered-structure compound is selected from a compound of the formula Bi.sub.2 A.sub.m-1 B.sub.m O.sub.3m+3 (wherein A is selected from Na, K, Pb, Ca, Sr, Ba and Bi; and B is selected from Fe(III), Ti, Nb, Ta, W and Mo; and m is a positive integer). The crystal structure of the bismuth layered-structure compound is such that the (Bi.sub.2 O.sub.2).sup.2+ layer and the (A.sub.m-1 B.sub.m O.sub.3m+1).sup.2- layer are alternately stacked. In other words, the basic crystal structure of the compound is such that the layered perovskite layer having a series of perovskite lattices of (m-1)ABO.sub.3 is sandwiched from above and below by (Bi.sub.2 O.sub.2).sup.2 +layers. Here, it is not always the case that the elements A and B to be selected are single elements.
Examples of such bismuth layered-structure compound materials include SrBi.sub.2 Ta.sub.2 O.sub.9, SrBi.sub.2 Nb.sub.2 O.sub.9, Bi.sub.4 Ti.sub.3 O.sub.12, BaBi.sub.2 Nb.sub.2 O.sub.9, BaBi.sub.2 Ta.sub.2 O.sub.9, PbBi.sub.2 Nb.sub.2 O.sub.9, PbBi.sub.2 Ta.sub.2 O.sub.9, SrBi.sub.4 Ti.sub.4 O.sub.15, PbBi.sub.4 Ti.sub.4 O.sub.15, Na.sub.0-5 Bi.sub.4.5 Ti.sub.4 O.sub.15, K.sub.0.5 Bi.sub.4.5 Ti.sub.4 O.sub.15, Sr.sub.2 Bi.sub.4 Ti.sub.5 O.sub.18, Ba.sub.2 Bi.sub.2 Ta.sub.5 O.sub.18, Pb.sub.2 Bi.sub.4 Ti.sub.5 O.sub.18 and the like.
The method for manufacturing a thin ferroelectric film may be a physical method such as vacuum vapor deposition method, sputtering method and laser abrasion method, or a chemical method such as sol-gel method, MOD (Metal Organic Decomposition) method or MOCVD (Metal Organic Chemical Vapor Deposition) method employing thermal decomposition and oxidation of an organic metal compound used as a starting material to produce oxide ferroelectrics.
Among the above-mentioned methods for forming a ferroelectric film, the MOCVD method provides an excellent step-coverage and also may possibly be used for low temperature film formation, so that the MOCVD method is promising in view of manufacturing highly integrated FRAMs and has recently been under active research and development.
On the other hand, the sol-gel method or the MOD method has been widely used owing to the fact that a uniform mixture in an atomic level can be obtained, that the composition can be controlled easily and the reproducibility is excellent, that no special vacuum apparatus is required and a film having a large area can be formed under ordinary pressure and that the industrial cost is small.
Especially, the MOD method is used for forming the above-mentioned thin film of bismuth layered-structure compound, and a thin ferroelectric film or a thin dielectric film is manufactured through the following steps in the film formation process according to conventional MOD methods (International Application No. PCT/US92/10542, PCT/US93/10021).
(1) Step of applying a precursor solution containing a composite alkoxide and the like onto a substrate by spin coating method or the like for forming a film;
(2) Step of annealing and drying the obtained film at 150.degree. C. for 30 seconds to several minutes for removing, from the film, the solvent and the alcohol and residual water that have been generated by the reaction of step (1);
(3) Step of annealing the film at 725.degree. C. for 30 seconds under oxygen atmosphere by employing a RTA (Rapid Thermal Annealing) method for removing the organic components in the film by thermal decomposition; and
(4) Step of annealing the film at 800.degree. C. for one hour under oxygen atmosphere for crystallization of the film;
(5) Step of annealing the film at 800.degree. C. for 30 minutes under oxygen atmosphere after an upper electrode is formed.
Here, in order for obtaining the desired film thickness, the steps of (1) to (3) are repeated and, finally, the steps of (4) and (5) are carried out.
The thin ferroelectric film or thin dielectric film is thus fabricated.
However, by a method of manufacturing a thin ferroelectric film using the above-mentioned conventional MOD method, little crystallization of the thin ferroelectric film takes place at annealing temperature of 650.degree. C. or less. Accordingly, in order to obtain a high residual polarization, it is necessary to carry out an annealing step at an extremely high temperature of 800.degree. C. for a period of time as long as one hour (International Application No. PCT/US93/10021). Therefore, in forming a thin ferroelectric film element on an integrated circuit having a stack structure, there will occur damages such as poor contact and deterioration in characteristics due to interdiffusion and oxidation between the viahole (contact hole) material and the electrode material, thus placing a hindrance particularly in manufacturing such highly integrated devices.
Also, since the annealing temperature is thus high, the particle diameter of the crystal particles constituting the thin ferroelectric film is as large as 1000 to 2000 .ANG. and the irregularity on the surface of the thin film is large. Accordingly, it has not been possible to apply the conventional MOD method to fine submicron processing which is required in manufacturing highly integrated devices.
Moreover, in the case of highly integrated FRAMs of 4M bit to 16M bit or more, the capacitor area will be small and the spontaneous residual polarization Pr required in the ferroelectric materials will be large, so that Pr of at least 10 .mu.C/cm.sup.2 will be necessary. In the case of the thin SrBi.sub.2 Ta.sub.2 O.sub.9 film, the spontaneous residual polarization Pr will be small in accordance with the decrease in the annealing temperature, so that it has not been possible to obtain sufficient Pr required in highly integrated FRAMs by conventional methods if the annealing temperature is lowered.
On the other hand, it is known in the art that Nb is added so as to increase Pr of the thin SrBi.sub.2 Ta.sub.2 O.sub.9 film. However, if Nb is added into the thin SrBi.sub.2 Ta.sub.2 O.sub.9 film, the coercive field Ec will be large although Pr will certainly be large. Accordingly, the leakage current will increase in addition to the rise in the operation voltage and, moreover, the fatigue characteristics will be deteriorated.